ackgroud Info:
-- 3 gallon tank
-- 1 betta and 1 mystery snail (mystery snail has been removed for 3 days)
-- heated to 80 degrees
-- 100% water change every 3 days. I rinse all tank contents(heater, decor, gravel, tank walls) with hot tap water before refilling with conditioned and aged water and I also match the water temperature to original temp. before adding the betta back in.
-- water params are always perfectly spot on

Problems:
1. For he past 2 weeks I have noticed a film on the water surface of my betta's tank. On water change day (every 3 days) the film starts to separate into spots. 3 days ago, I removed the snail that has been living with the betta since dec. 9th -- I am still getting the film after removing the snail and thoroughly rinsing all contents of the tank.

2. Over the past 2 weeks as well, the betta's fins have been deteriorating. At first I thought he was tail biting but since he also has pinholes and separation in the fins I'm not sure.

I don't understand how this is happening because I keep my betta's tanks very, very clean. I have another betta in the same conditions with the exceptions of different gravel and never having a snail, and his water is always clear and he is doing fine..

I added aqua culture's natural gravel I got from Wal-Mart the same time I added the snail.

I took a VERY long time for the water to run clear when rinsing the gravel for the first time...

add a bubbler to get some surface agitation to break up the bio-film. The betta might be stressed from not being able to reach the surface enough. See if theres any improvement. If not dose with anti bacterial meds like maracyne. Run a nano filter after with carbon to get rid of the meds as well as frequent water changes. For long term growth, consider adding floating plants to keep the water cleaner and keep biofilms away.